Such a filter is based on the following principles:
using the modes of a metal-clad dielectric resonator; and PA0 reusing conventional excitation methods and coupling modes, in particular by virtue of adjustment screws acting on the electric field and coupling irises using the magnetic field. PA0 the position of the resonator in the metal cavity; PA0 the material used to make the cavity, the adjustment devices, and the resonator holding system; and PA0 the principle whereby the dielectric resonator is held within the cavity. PA0 utilization of invar and carbon fiber; or PA0 utilization of some other material with the coefficients of expansion used being compensated. PA0 low loss insulating materials (polystryene or PTFE (polytetrafluorethylene), foam) in the form of a column or a cushion. PA0 it makes it possible to obtain a sufficient frequency difference between the HE.sub.1,1.delta. mode, for example, and the other modes to make it possible to obtain a fairly wide band which is free of parasitic modes; PA0 the coupling slots remain small in size, thereby greatly reducing parasitic coupling; and PA0 it makes it possible to design filters with a more than two-mode structure. PA0 the cavity is closed at at least one of its ends by an iris; and PA0 the resonator is held by a system including a mandrel fixed to the inside wall of the cavity by three spokes at 120.degree. intervals, with one of them constituting a fixed point and with the other two leaving play between the cavity and the mandrel system via resilient seals for absorbing any radial stress due to differential expansion. PA0 a first part terminating at a first longitudinal end in a conical inside bearing surface and at its second end in a threaded inside bearing surface; PA0 a second, moving part situated inside said first part and having an outer sloping bearing surface suitable for sliding against the conical bearing surface; and PA0 a threaded part engaged in the threaded bearing surface of the first part. PA0 to optimize the Q-factor (no lossy material in the critical zones); and PA0 to obtain a temperature stability coefficient of about 4 ppm/.degree.C. (where ppm means parts per million), which can be compensated by using a dielectric resonator having a -4 ppm/.degree.C. characteristic.
An article published in "Electronics Letters", vol. 16, No. 17, August 14, 1980, pp. 646-647, entitled "Dielectric resonator dual mode filter" by P. Guillon, Y. Garault, and J. Farenc describes a metal-clad dielectric resonator which is cylindrical in shape and in which a plurality of degenerative modes at identical natural frequencies can propagate. These degenerate modes may be mutually coupled in order to form coupled circuits, by disturbing the geometrical configuration of the structure. It is thus possible to disturb the frequency for the TE.sub.01p mode by means of a tuning screw, together with a two-mode dielectric filter using two HE.sub.111 modes of the resonator which are polarized perpendicularly to each other.
European patent application No. 0 064 799 describes a ceramic resonator element disposed in a cavity in order to form a composite microwave resonator. Two tuning screws situated in the cavity along orthogonal axes serve to tune the assembly along said axes to frequencies close to the fundamental resonance frequency of the resonator element. A plurality of cavities of this type may be assembled together to form a waveguide by using a plurality of transverse separations. Coupling between these various cavities can then be provided by means of single slots, by a pair of slots in a cross configuration, or by using circular irises. An adjustment screw is positioned in each cavity along an axis at 45.degree. relative to the oorthogonal tuning screws in such a manner that resonance along each of the orthogonal axes is coupled to resonance along the other axis.
However, these prior art documents give no details concerning:
In other prior art documents, various details are given concerning the materials used for the cavity:
As for the holding dielectric, few precise solutions are given, e.g.:
While ensuring that it is possible to optimize the response curves of filters having dielectric resonators in a band close to resonance and over a wide band, the invention has the object of solving various questions raised in the manufacture of such filters.